The present invention relates to impact printers and more particularly to a novel design for obtaining a non-linear spring force which is advantageous for use in matrix type printing solenoids.
Dot matrix printers are typically comprised of a plurality of solenoid driven print wires mounted within a movable print head assembly which traverses an impression material such as a paper document. During movement of the print head across the paper document, solenoids are selectively energized to drive their associated print wires either against an inked ribbon and ultimately against the paper document or directly against the paper document, to form dot column patterns at closely spaced intervals along the printing line. In a typical dot-matrix printer, a 5 .times. 7 dot matrix is formed for each character by a print head using a substantiallly vertical row of 7 solenoid driven print wires, which print row successively forms 5 dot columns to collectively form a single character symbol or segmented pattern. Selective energization of the solenoids permits alphabetic and numeric characters, punctuation symbols, segmented patterns, and the like to be generated.
In order to achieve high printing speeds, the print wire must be accelerated from a rest position to a velocity sufficiently high to form a high-contrast dot on the original document and, typically, five carbon copies, and return to its original rest position in a total elapsed time less than one millisecond. It is impractical to obtain faster operating speeds using present day conventional solenoid designs. Significantly faster operating speeds have been obtained using a solenoid design, such as described in U.S. Patent Application Ser. No. 499,632, filed on Aug. 22, 1974, and assigned to the assignee of the present invention, in which a case houses an annular-shaped solenoid coil having a hollow core and a cylindrically-shaped magnetic armature with its rearward portion positioned within the hollow core of the solenoid winding and a slender reciprocating print wire attached to its frontward position and extending through an elongated axial opening in the solenoid coil. The rear end of the armature extends beyond the rearward end of the solenoid winding and terminates in a headed portion selectively abutting two or more linear springs. The outer periphery of each linear spring rests upon a surface of an annular-shaped ring assembly shaped in a stepped arrangement such that upon energization of the solenoid coil the armature is accelerated towards impact velocity rapidly overcoming the biasing force of the first spring (of light spring force) and causing a second spring (of greater spring force) to engage a lower step after significant axial movement of the print wire assembly in the print direction, whereby the armature rapidly returns to the rest position after deenergization of the solenoid. While this design reduces the elapsed time between acceleration of the armature from the rest position to the time when the armature returns to the rest position by approximately one-half the elapsed time found in a single spring solenoid assembly, the requirements for multiple spring members and their precise alignment and attachment to the armature header lead to greater manufacturing and assembly time and costs therefor.